The Carbon Cycle

The carbon cycle is a biogeochemical cycle that represents the exchange of carbon (C) between different reservoirs on Earth, such as the atmosphere, biosphere, oceans and rocks. It is a critical part of the Earth system that is required to sustain life and regulate climate. The carbon cycle can be subdivided into a short-term cycle and a long-term cycle, which operate on different spatial and temporal scales.

Short-term carbon cycle
The short-term cycle involves carbon transfer between the atmosphere, biosphere and oceans on timescales of days to tens of thousands of years. Carbon dioxide (CO2) from the atmosphere is taken up by plants on the continents or by phytoplankton in the oceans through photosynthesis, during which it is transformed into organic matter and oxygen (O2). Over time, plants and phytoplankton may be eaten by higher organisms, but eventually their organic matter will be respired. Respiration is the reverse process of photosynthesis and consumes O2 and produces CO2, which is often associated with microbes. The processes of photosynthesis and respiration may be expressed as follows:

CO2 + H2O <-> CH2O + O2

On land, carbon may also be transferred to soils by the falling of leaves, the death of plants and the development of soil biota. This soil carbon may is subsequently transported to the oceans in dissolved form by rivers. Gas exchange between the oceans and atmosphere completes the short-term carbon cycle.

Long-term carbon cycle
The long-term carbon cycle involves carbon transfer between the atmosphere, biosphere, oceans and rocks on timescales of millions of years. It consists of two subsycles: the silicate-carbonate subcycle and the organic subcycle.

In the silicate-carbonate subcycle, silicate rocks on the continents (CaSiO3) are subjected to chemical weathering over time, which consumes CO2 from the atmosphere. Rivers deliver the dissolved minerals to the oceans, where they are reprecipitated by biological activity as marine silicates and carbonates (SiO2 and CaCO3, respectively) and eventually buried in the geological record. These sedimentary rocks are ultimately returned to Earth’s surface through subduction, volcanism and metamorphism, which releases CO2 back to the atmosphere and results in the formation of new silicate rocks. These processes may be expressed as follows:

CaSiO3 + CO2 <-> CaCO3 + SiO2

In the organic subcycle, photosynthesis and respiration result in carbon transfer on geological timescales. Organic matter is eventually buried in sedimentary rocks, such as shales and coals, which consumes CO2 from the atmosphere. Old sedimentary rocks enriched in organic matter may become subjected to chemical weathering on the continents over time, which releases CO2 back to the atmosphere.

The long-term carbon cycle is able to exchange massive amounts of carbon between rocks and the other reservoirs on geological timescales. As a consequence, it governs atmospheric CO2 concentrations and regulates global surface temperatures and climate. Moreover, major perturbations of the carbon cycle are shown to have greatly affected life on Earth in the geological past, for example during mass extinctions.

Information source: Berner, R. A. (2004). The Phanerozoic Carbon Cycle: CO2 and O2. Oxford University Press.

Image: Paraná pines at sunrise in Serra da Bocaina National Park, Brazil. Credit: Heris Luiz Cordeiro Rocha, Wikimedia Commons.